The present invention relates to an optical system used for an image projection apparatus using an image-forming element such as a liquid crystal panel.
In such an image projection apparatus, it is important that the apparatus is capable of projecting a bright image with a high contrast and a substantially even brightness over the entire image.
An illumination optical system for an image projection apparatus (a projector) using a reflective image-forming element such as a reflective liquid crystal panel generates a substantially parallel luminous flux by reflecting a luminous flux from a light source by a parabolic reflector. This parallel luminous flux is divided and collected by a first fly-eye lens, and each of the divided luminous fluxes forms an image of the light source (a secondary light source image) at the vicinity of a second fly-eye lens. Each of minute lenses (lens cells) constituting the fly-eye lens has a rectangular shape similar to the image-forming element that is an illuminated surface.
The divided luminous fluxes emerging from the second fly-eye lens are condensed by a condenser lens to be superposed with each other and illuminate the image-forming element through a color separation/combination optical system. The color separation/combination optical system uses an optical element having a dichroic film or a polarization beam splitting film (a dichroic prism or a polarization beam splitter).
In such an image projection apparatus, increasing use efficiency of light from the light source generally makes an angular distribution of a luminous flux large. Thus, when using an optical element with a sensitive angular characteristic, that is, an optical element having a dichroic film or a polarization beam splitting film that is inclined with respect to an optical axis in the color separation/combination optical system, deterioration of image quality is caused such as unevenness of brightness (unevenness of color) or reduction of contrast.
FIGS. 20 and 21 show examples of angular dependent characteristics (transmittance characteristic and reflectance characteristic) of a polarization beam splitting film, respectively. The characteristics for light incident on the polarization beam splitting film at an angle of 47 or 49 degrees are lower than those for light incident thereon at an angle of 45 degrees, and the lowering degree of the characteristics increases as a shift of the incident angle from 45 degrees becomes larger.
This lowering of the characteristics causes so-called leakage of light, which decreases the contrast. When using the polarization beam splitting film, a shift of the incident angle causes unevenness of color (change of color), thereby displaying an image with a color different from that originally expected.
Japanese Patent Laid-Open No. 7-181392 has disclosed an asymmetric optical system in which, to prevent such deterioration of image quality, an angular distribution of a luminous flux incident on an optical element is small in a direction (a sensitive direction) that the optical element is sensitive to the angular distribution and an angular distribution thereof is large in a direction (an insensitive direction) that the optical element is insensitive to the angular distribution.
In the optical system disclosed in Japanese Patent Laid-Open No. 7-181392, lens cells constituting a first fly-eye lens are decentered such that the first fly-eye lens has a positive optical power as a whole in the sensitive direction.
In addition, in the same direction, lens cells constituting a second fly-eye lens are decentered such that the second fly-eye lens has a negative optical power as a whole. Such compression of the luminous flux from a reflector only in the sensitive direction makes the angular distribution of the luminous flux incident on the optical element asymmetric between in the sensitive direction and in the insensitive direction.
However, the compression of the luminous flux from the reflector only in one of cross-sections including the optical axis cannot reduce the angular distribution of the luminous flux incident on the polarization beam splitting film in the other cross-section (the cross-section perpendicular to the one cross-section), so that an improvement of the image quality is difficult. Further, since reduction of the size of the reflector for decreasing the diameter of the luminous flux makes a projected image dark, a certain degree of a large outer diameter of the reflector is required for ensuring brightness.